On ACT English, a comma belongs between items in a list of three or more, and between two adjectives only when you could swap them. A comma that is not doing one of those jobs is usually the wrong choice. The broader comma jobs are in ACT comma rules.
Lists of three or more
She packed granola, a water bottle, and two pencils. Each item gets separated, including the comma before and. Two items are not a series: granola and a water bottle needs no comma. If the list is only a pair of verbs with one subject, skip the comma too: She wrote the essay and revised it.
Two adjectives
Put a comma between adjectives when and would sound fine. A long, boring lecture works, because it is long and boring. A red sports car does not, because “red and sports” is not how anyone talks. Sports names the kind of car; red describes that kind. No comma.
Opening clauses use a different comma. That rule is in commas after introductory clauses. The rest of the punctuation set is in ACT punctuation rules and the grammar rules the section actually tests.
The bottom line
Count the items. Three or more get commas between them. Two adjectives get a comma only if you can reverse them.
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